In oil-in-water emulsion cosmetic compositions such as ointment, cream, milky lotion and serum, a polyoxyethylene-based surfactant has been used. Examples of the polyoxyethylene-based surfactant include polyoxyethylene sorbitan fatty acid esters, polyethylene glycol fatty acid esters and polyoxyethylene alkyl ethers.
These polyoxyethylene-based surfactants are used because these surfactants have various HLB values and therefore it becomes possible to widely adjust the HLB value by using them in combination, resulting in wide applications of the surfactant.
However, it has been known that this polyoxyethylene-based surfactant has some fear for safety to the human body and high concentration of the polyoxyethylene-based surfactant causes irritation of the skin. Hydrophilicity and lipophilicity of the polyoxyethylene-based surfactant easily vary with the temperature and an oil-in-water emulsion cosmetic composition obtained by using the same deteriorates by temperature variation.
As a surfactant which is excellent in safety as compared with a polyoxyethylene-based surfactant and is free from discomfort of flavor or odor, for example, a polyglycerin fatty acid ester used widely in foods has recently been used in a cosmetic composition (Japanese Unexamined Patent Publication, First Publication No. Sho 58-185537). It has commonly been known that hydrophilicity and lipophilicity of the polyglycerin fatty acid ester hardly vary with temperature variation and thus an oil-in-water emulsion cosmetic composition obtained by using the same is also stable against temperature variation.
It is considered that various stabilities of the oil-in-water emulsion cosmetic composition depend on stability in an emulsified (state) (hereinafter referred to as “emulsion stability”) of a cosmetic composition.
To the contrary, when an oil-in-water emulsion cosmetic composition having poor emulsion stability is exposed under severe conditions during a circulation process or use by a consumer, there arises a problem that thickening, solidification, separation and aggregation may occur and thus commercial value may be lost. There also arises a problem that an emulsion cosmetic composition is thickened or solidified by temperature variation during storage of the product or shaking during transportation, thus resulting in separation and aggregation.
As described above, the oil-in-water emulsion cosmetic composition obtained by using the polyglycerin fatty acid ester had a problem how emulsion stability is enhanced.
To enhance emulsion stability, an approach of improving the surfactant of the oil-in-water emulsion cosmetic composition and an approach of improving the emulsification method of the oil-in-water emulsion cosmetic composition have been made and various emulsification methods have been developed.
An emulsification method, which has been employed most popularly, is a method in which an oil phase containing a surfactant is added in an aqueous phase and the oil phase is emulsified by a mechanical shear force of an emulsifying machine such as homomixer (hereinafter referred to as a “dispersion emulsification method”). However, according to this method, the functions of the surfactant do not efficiently exert on interface between oil and water. With the increase of the level of quality required to cosmetics, various formulations in a cosmetic composition have been developed and it becomes difficult to decrease the particle size of emulsified particles. There is a limit in the development of the emulsifying machine, and the development of a new emulsification method was made. Therefore, a phase inversion emulsification method (reverse emulsification method) was developed as the emulsification method using a surface chemical technique, not a technique due to a mechanical force.
The phase inversion emulsification method is an emulsification method which requires to match a HLB value of the surfactant to be added to a required HLB value of an oily component, and is specifically a method comprising dissolving or dispersing a surfactant having almost the same HLB value as the required HLB value of an oily component in the oily phase, adding an aqueous phase to obtain a water-in-oil emulsion cosmetic composition (W/O emulsion cosmetic composition), and adding furthermore an aqueous phase thereby to convert into an oil-in-water (O/W) emulsion cosmetic composition through phase inversion, thus obtaining an oil-in-water emulsion cosmetic composition (O/W emulsion cosmetic composition). In the phase inversion emulsification method, a mixture of two or more kinds of surfactants is commonly used.
When the phase inversion emulsification method is used, it is possible to obtain an oil-in-water emulsion cosmetic composition having a small emulsified particle size as compared with a dispersion emulsification method in which an oil phase is added to an aqueous phase to produce an oil-in-water emulsion cosmetic composition. Therefore, it is commonly used in the production of a cosmetic composition.
It is reported that the oil-in-water emulsion cosmetic composition having a small emulsified particle size can be obtained by the following reason. That is, when the required HLB value of the oily component and the HLB value of the surfactant are adjusted to almost the same value, emulsified particles are produced via a lamellar liquid crystal phase in case of phase inversion (see 1981, “Petrochemistry”, Vol. 30, No. 1, pp. 38-43). As a detailed mechanism, it is made clear that submicron-sized emulsified particles are produced through the following steps: W/O emulsion cosmetic composition→lamellar liquid crystal phase→O (oil phase)/D (surfactant phase)→O/W emulsion cosmetic composition in the process of adding an aqueous phase to an oil phase (see 1981, “Petrochemistry”, Vol. 30, No. 1, pp. 38-43).
Examples of the emulsification method employing a surface chemical technique include, in addition to this phase inversion emulsification method, an emulsification method employing a surface chemical technique, such as D phase emulsification method or liquid crystal emulsification method. However, in the emulsification method employing a surface chemical technique, for example, a polyhydric alcohol must be added as an essential component in case of emulsification. Therefore, the product is sensorily restricted, that is, appearance and formulation are restricted and it requires it takes a long time to previously study due to a phase diagram. With respect to load in the production, there were drawbacks that a cosmetic composition can not be produced according to the formulation in which a polyhydric alcohol gel must be produced and long time is required to emulsify, and also it is impossible to add in the amount enough to uniformly disperse by an agitator an oil phase before emulsification and a polyhydric alcohol, that is, the formulation in which the oil phase and polyhydric alcohol are contained in a small amount.
Therefore, there is proposed a method of decreasing the particle size of emulsified particles, without adding the essential components or taking long time in case of emulsification, by adopting a surface chemical technique employing the above-described phase inversion emulsification step in the dispersion emulsification method in which an aqueous phase is added to an oil phase and emulsification is conducted by a mechanical force, that is, a dispersion emulsification method employing self-emulsification (hereinafter referred to as a “self-emulsification method”). This method is a method, in which, when an oily component containing a surfactant having self emulsification characteristics added therein is contacted with water, self-emulsification based on the step of the phase inversion emulsification rapidly occurs and an oil-in-water emulsion cosmetic composition having a small emulsified particle size can be produced. The oil-in-water emulsion cosmetic composition obtained by using such a self-emulsification method has an advantage that it can overcome defects in the development process or production and is excellent in commodity.
It is generally known that such an emulsification method employing self emulsification characteristics of the surfactant can be applied in a polyoxyethylene-based surfactant. However, in this case, it is necessary to add a polyoxyethylene-based surfactant in a large amount of 100 to 120% by mass based on 100% by mass of the oily component so as to rapidly cause self-emulsification when contacted with water. However, it has been known that this polyoxyethylene-based surfactant has some fear for safety to the human body and high concentration of the polyoxyethylene-based surfactant causes irritation of the skin, as described above. The oil-in-water emulsion cosmetic composition obtained by this method using this polyoxyethylene-based surfactant has a drawback such as poor tactile sensation.
As a method of emulsifying an oil-in-water emulsion cosmetic composition obtained by using the polyglycerin fatty acid ester, for example, a D phase emulsification method and a liquid crystal emulsification method have recently been used, in addition to a conventional dispersion emulsification method, and also a phase inversion emulsification method, which could have not been used, has recently been used. However, the method except for these dispersion emulsification methods has a problem such as restriction of the product and a problem in the production.
To the contrary, when the self-emulsification method is applied to an oil-in-water emulsion cosmetic composition obtained by using a polyglycerin fatty acid ester, a polyglycerin fatty acid ester is not easily dissolved in an oily component, and if it is dissolved in the oily component, the composition is not self-emulsified and is emulsified only by a mechanical force in a homomixer. Therefore, the resulting oil-in-water emulsion cosmetic composition had poor emulsion stability. Therefore, the oil-in-water emulsion cosmetic composition produced by the dispersion emulsification method employing self-emulsification using a polyglycerin fatty acid ester was not circulated in the market. The diglycerin monofatty acid ester having self emulsification characteristics is circulated but has poor self emulsification characteristics, and thus a satisfactory emulsion has never been obtained.